Cell-based therapies have shown enormous promise in reducing neurological deficits associated with stroke. One of the most effective of these therapies is bone marrow stromal cells (MSCs), that has been demonstrated to be highly neurorestorative. In this application, we will investigate the mechanisms by which MSCs produce this neurorestorative effect. Our preliminary data strongly indicate that MSC treatment of stroke promotes neurite remodeling of brain. We propose that when administered after stroke, MSCs activate tissue plasminogen activator (tPA) within parenchymal cells, and tPA mediates neurite remodeling leading to improvement in neurological function. Therefore, the following three hypotheses are tested: Hypothesis 1: a) MSCs increase tPA activity in parenchymal cells; b) Increased tPA activity increases neurite remodeling; c) Increased neurite remodeling contributes to improvement of functional outcome after stroke. Hypothesis 2: a) MSCs up-regulate tPA activity in astrocytes, neurons and endothelial cells via the Shh signaling pathway; b) MSCs down-regulate TGF-1/PAI-1 via the Shh signaling pathway and thereby increase tPA activity. Hypothesis 3: tPA activity increased by MSCs promotes neurite remodeling via plasmin-dependent proteolytic cleavage of pro-neurotrophins: pro-nerve growth factor (pro-NGF) to NGF, pro-brain derived neurotrophic factor (pro-BDNF) to BDNF These hypotheses dissect the interactions of exogenous MSCs and endogenous parenchymal cells and their affect on tPA activity, neurite remodeling and neurological function after stroke. Our studies employ genetically modified tPA-/-, Plg-/-mice as well as an array of novel and well-established experimental techniques in our laboratory. To our knowledge, our work is the first to investigate tPA activity as a key unifying factor to amplify beneficial actions of exogenous cells in the CNS. This project is a coherent and highly interwoven effort to elucidate the molecular and cellular pathways by which injured brain can be remodeled by cell-based therapies. Our ultimate goal is to delineate the mechanistic underpinnings of cell-based therapy in the restorative treatment of stroke. The therapeutic implications of our studies for all neurological disease and injury are evident.